In a conventional forming method of a resistor between electrodes, a photo-resist having an opening for adjusting a resistance value is formed on a resistive area. Through the opening, the resistive area is etched so that a desirable resistance value is obtained by monitoring the resistance value.
The above conventional manufacturing method is described by referring to the cross sections in FIG. 3(A1) through FIG. 3(F1) and the plan views in FIG. 3(A2) through FIG. 3(F2).
First, as shown in FIG. 3(A1) and FIG. 3(A2), a photo-resist 3 having an opening 2 is formed on a semi-insulating substrate 1 such as GaAs, then an ion-plantation of an impurity is conducted into the semi-insulating substrate 1 in order to form a resistive contact area 4 of high impurity concentration.
Next, as shown in FIG. 3(B1) and FIG. 3(B2), the photo resist is removed, and a new photo resist 6 having an opening 5 is formed on the semi-insulating substrate 1. Then, ions are planted into the semi-insulating substrate 1 in order to form a resistive area 7 of low impurity concentration.
As shown in FIG. 3(C1) and FIG. 3(C2), the photo-resist 6 is removed and a photo-resist 9 having an opening 8 for forming an electrode is formed on the semi-insulating substrate 1.
As shown in FIG. 3(D1) and FIG. 3(D2), after metallic film being formed, the photo resist 9 is removed, and an ohmic electrode 10 is formed by Lift-Off method.
As shown in FIG. 3(E1) and FIG. 3(E2), a photo-resist 12 having an opening 11 across the entire resistive area 7 is formed. The opening 11 is used for adjusting a resistance value. Then the resistive area 7 is etched, and the resistance value is adjusted by monitoring a value thereof in order to get a desirable resistance value.
Finally, as shown in FIG. 3(F1) and FIG. 3(F2), the photo-resist 12 is removed in order to form a resistor between electrodes, of which resistance value has been adjusted to a desired value.
In the above method, when adjusting the resistance value by etching the resistive area 7 through the opening 11 which is across the entire resistive area 7 with monitoring the resistance value, the resistance value sharply increases after a certain period of etching, as shown in FIG. 4. Accordingly, if an etching is slightly overtimed, the etching would penetrate the resistive area 7, whereby the resistance value becomes infinite.
The etching time for adjusting the resistance value within a tolerance of a designed value, as shown in FIG. 4, is on the order of seconds. Therefore, it is difficult to control the resistance value, and the wide range of the resistance values resulting from the foregoing process contributes to lowering the yield rate.
As a result, there is a need to be able to control the resulting resistance value of the resistor formed between electrodes as described above so as to increase the yield rate when forming the resistance in an integrated circuit.